Light-emitting diodes (LEDs) emit light when a forward current is passed through them. The light intensity, which may be referred to as the luminous flux, radiant flux, or simply the lumens, output by an LED is proportional to the forward current. As with most diodes, LEDs have a forward voltage, which, in ideal circumstances, the forward voltage remains constant, so the light intensity is proportional to the forward current, which will also be constant. Therefore, in ideal circumstances, the intensity of light output by an LED is very predictable and constant because it is related to a constant forward current.
In real circumstances, the forward voltage of an LED is not constant over time or from one LED to another. For example, as an LED ages, its forward voltages may decrease. Likewise, as the temperature of an LED increases, its forward voltage decreases. When the forward voltage of an LED decreases and the forward current is maintained constant, the light intensity emitted by the LED decreases. The opposite occurs when the forward voltage of an LED increases. With a constant forward current, more power is delivered to the LED and the intensity of light emitted by the LED increases.